The present invention relates to a toolholder device for a machine tool and more particularly toolholder spindles for valve-seat grinders.
According to the patent EP-C-No. 22,796, the toolholder spindle described is integral with a table moveable in a plane in two directions perpendicular to one another and comprises:
a toolholder spindle; PA1 a means for the angular orientation of the spindle relative to the plane, formed by a sleeve with two bearing surfaces, one upper and the other lower, of convex sphericity, and by a housing with a lower seat of matching concave sphericity; PA1 the sleeve being capable of being successively locked and released relative to the housing by means of an upper seat moveable relative to the housing; PA1 and means for supporting the sleeve relative to the housing without friction during the release phase, in the form of a gas stream providing a cushion between the sleeve and the seats mentioned above.
Such a spindle, because of its low inertia, can be positioned accurately, and its effectiveness in this respect has been shown in many uses, particularly for valve-seat grinding machines.
Nevertheless, such a spindle seems more suitable for the machining of workpieces by the unit than for large-series operations or jobs because of the limitations explained below.
First of all, in an industrial environment, the technique of a gas cushion is usually put into practice by providing, between the sleeve and the housing, an open air circuit obtained from a compressed-air source and from calibrated leaks towards the outside. Consequently, if a closed circuit of a suitable gas (for example, nitrogen) is relinquished, it seems difficult to obtain a cushion of constant thickness, especially because of the temperature variations which affect the supporting air and which change the pressure and/or flow of the latter. Moreover, because condensates or moisture are present in the available compressed air, it is necessary to use additional filtration or condensation devices. For all these reasons, it appears difficult, in practice, in an industrial environment to control the air cushion in the critical region between the sleeve and the housing.
Secondly, the spindle combined with its mechanism for driving the rotation represents a relatively large mass. From the moment when the spindle is positioned relative to the workpiece to be machined, the inertia of this assembly in relation to the absence of any appeciable friction between the sleeve and the housing generates oscillations of the workpiece which are of decreasing amplitude, until the desired state of equilibrium is reached; this results in a relatively long time for setting up the spindle, and this is often incompatible with large-series machining operations. Or if machining, for example grinding, begins before the state of equilibrium of the spindle is reached, it will be impossible to achieve the desired machining accuracy.
The present invention proposes to overcome all these disadvantages, and, particularly for a toolholder spindle, such as that defined above, it looked for another way of supporting the sleeve relative to the housing, allowing large-series machine operations to be carried out in an industrial environment.
According to the invention, and in general terms, means for generating a magnetic field, for example a magnet, are associated with the lower seat of the housing, and control means make it possible to push the lower bearing surface of the sleeve into the position of angular adjustment of the spindle and draw the same bearing surface into a working position of the said spindle.
These means used for supporting a spherical member are known per se and were described, for example, in the U.S. Pat. No. 4,461,463 for orienting a plane surface angularly relative to a plane reference surface, and in the Patent FR-A-No. 2,535,479 for orienting two parts of a space vehicle relative to one another according to three degrees of freedom.
The use of similar means for a toolholder spindle according to the invention gives the latter results or performances which are significant, novel per se and unexpected.
As a result of the present invention, the mechanical arrangement of the toolholder spindle becomes much simpler. In fact, there is no longer any need to use an upper seat, moveable in the manner of a piston relative to the housing, in order to lock the sleeve and therefore the spindle in their working positions. It is sufficient, for this purpose, to cut off the magnetic field or reverse it in order to "stick" the sleeve to its seat.
Moreover, if the magnetic field is regulated in the opposite direction to the oscillations of the sleeve and spindle, it becomes possible to stabilize the latter immediately in its position of equilibrium.
In the same way, it becomes possible to adjust the air gap between the sleeve and the seat and therefore adapt the thickness of the cushion between these two components to the type of machining operation to be carried out.
Likewise according to the invention, the means of driving the spindle in rotation are incorporated within the housing in the form of an electrical stator located in the region of the seat or inside the sleeve, and the spindle or the sleeve connected to the spindle in terms of rotation forms a rotor.
If all the characteristics of the present invention are put into practice, the overall result is a toolholder device, the mechanical arrangement of which becomes very simple.